The present invention relates generally to gas discharge devices, especially those utilized in ring laser angular rate sensors, and more particularly, to the cathode construction found in such assemblies.
Ring laser angular rate sensors are well known and are particularly described in U.S. Pat. No. 3,373,650, issued to Killpatrick, and U.S. Pat. No. 3,390,606, issued to Podgorski, both of which are assigned to the assignee of the present invention. The above referred to patents are incorporated herein by reference thereto.
Ring laser angular rate sensors of the type referred to utilize a substantially thermally and mechanically stable block having a plurality of interconnected gas containing tunnels in the shape of a triangle, a rectangle, or the like. At each intersection of a pair of interconnected tunnels is a mirror mounted on the block. This arrangement of mirrors and interconnected tunnels forms an optical closed-loop path. Further, at least one anode and one cathode are each mounted on the block and in communication with the gas. Each of the components, including the mirrors, anode, and cathode, are sealed to the block to form a gas tight seal by any one of a variety of techniques. The block is usually filled with a lasing gas such as a mixture of helium and neon. A sufficiently large electrical potential is applied between the anode and cathode to cause a discharge current therebetween which results in the production of a pair of counter-propagating laser beams within the block.
Ring laser angular rate sensors of the type just described require long life characteristics. The life of a ring laser angular rate sensor is dramatically affected by (i) the seal between components mounted on the block, and (ii) the cathode life. The present invention is directed to a cathode construction which will enhance laser life.
One example of a prior art cathode consists of a housing composed of aluminum coated with a thin layer of oxide. During the manufacturing process the layer of aluminum oxide is formed naturally by exposing the aluminum cathode to an oxygen plasma with the aluminum cathode connected as the cathode in a gas discharge device circuit. The thin layer of oxide is formed due to the pressure of oxygen and the heating effect of the plasma. Another cathode construction consists of a hollow cylinder made of aluminum having a layer of oxide formed thereon by means of anodization as shown and described in U.S. Pat. No. 4,007,431, issued to Abbink, et al.
It should be understood that it is important that the cathode be mounted on the block in such a way as to form a gas tight seal. Commonly, with laser blocks made of quartz, Cervit, Zerodur, and the like, the aluminum cathode is sealed to form a gas tight seal to the block by use of an indium (solder) seal well understood in the art. A problem with cathode construction and mounting techniques of the prior art is that differences between the thermal expansion coefficients of aluminum and the laser block can cause a breakdown of the gas tight seal resulting in laser failure. This is so since the block usually has an ultra low thermal expansion coefficient as compared to the aluminum.
To constrain the thermal expansion and movement of the cathode relative to the block and thereby reduce the potential for breaking the gas tight seal, an Invar constraining ring may be placed around the aluminum cathode and inertia welded or epoxied thereto. Although the Invar ring constrains the thermal expansion of the aluminum relative to the block, there still remains some thermally caused movement which adversely affects laser life.